Electroforming and electrodeposition of stress-free nickel from the sulfamate bath



United States Patent 3,374,154 ELECTROFORMING AND ELECTRODEPOSITION 0FSTRESS-FREE NICKEL FROM THE SULFA- MATE BATH Hugh L. McCutchen, Warwick,N.Y., assignor to The International Nickel Company, New York, N.Y., acorpoi-anion of Delaware No Drawing. Filed July 12, 1965, Ser. No.471,431 5 Claims. (Cl. 204-3) The present invention is directed to amethod for electrodepositing nickel and, more particularly, to a methodfor electroforming nickel to provide a nickel deposit having acontrolled stress level and good appearance.

In U.S. patent application Ser. No. 368,651, now US. Patent No.3,326,782, a nickel sulfamate plating bath and method forelectrodepositing nickel therefrom is described. The bath described inthe aforementioned patent application is characterized by a special highconcentration of nickel sulfamate, i.e., the concentration of nickelsulfamate in the bath is about 500 to about 700 grams per liter(g.p.l.). It is pointed out in the said patent application that veryhigh cathode current densities may be employed in the bath and that theinternal stress level in the deposit produced therefrom can becontrolled by means of bath temperature and cathode current density. Thespecial bath and method may also be employed to produce bright nickeldeposits by controlling the current density to relatively low levels. Inorder to produce deposits having substantially zero internal stress,i.e., having an internal stress level in the range of about 1,000 poundsper square inch (p.s.i.) compressive to about 1,000 p.s.i. tensile, thecurrent density may be correlated with bath temperature when the bathcontains about 550 to about 650 g.p.l. of nickel sulfamate as shown inthe following table:

Cathode current density-amperes per square foot to provide in deposit aninternal stress level of 1000 p.s.i. compressive to 1000 p.s.i. tensileBath Temperature, C.:

55 110 to 125 60 160 to 180 65 210 to 240 70 260 to 290 When the currentdensity employed is at a lower level than that given in the foregoingtable, the internal stress in the resulting deposit becomes morestrongly compressive. Deposits produced under the foregoing conditionsusing the bath described in the aforementioned US. patent applicationare not bright and it is found that when a bright deposit is desiredthat the current density should not exceed about 70 amperes per squarefoot (a.s.f.) in instances wherein no organic brightening agents areemployed. Whensuch current densities are employed to achieve a brightdeposit the stress level is strongly compressive, for example, being onthe order of about 14,000 p.s.i. compressive when the deposit is formedfrom a bath containing about 600 g.p.l. nickel sulfamate operated at 60C. using a cathode current density of 50 a.s.f. This is undesirable inmany applications, for example, in electroforming wherein it is desiredthat the internal stress level of the deposit be substantially zero toavoid dimensional changes in the deposit.

Nickel sulfamate plating baths according to the disclosure in theaforementioned patent application have been operated on a commercialscale in a number of installations with good success. It is found thatthe temperature-current density control needed to provide deposits ofsubstantially zero stress from the concentrated nickel sulfamate bathhave imposed undesirable limitations in certain areas. For example, inthe production of electrotypes using matrices made of certain plastics,it is required that the bath temperature not exceed about F. (49 C.) orthe dimensions of the matrix will change. Again, in the electroformingof complex shapes (as distinguished from fiat surfaces), it isundesirable to employ high cathode current densities since non-uniformthicknesses of electrodeposited nickel are then encountered.Furthermore, the high concentration of nickel sulfamate in the bathresults in high viscosity which apparently contributes to pitting in thedeposits. Other disadvantages flowing from operation of the said bath atthe required temperature levels of 55 C. (131 F.) or 60 C. F.) are anincreased bath hydrolysis rate and increased evaporation loss.

A need accordingly has arisen to provide a means whereby a deposithaving an internal stress level of substantially zero may be producedusing the current densities and bath temperatures conventionallyemployed in electroforming, i.e., current densities of the order ofabout 10 to about 100 a.s.f. and temperatures as low as 100 F. A methodfor electrodepositing nickel has now been discovered whereby nickel maybe electrodeposited from a sulfamate bath in the absence of specialorganic addition agents which will have a substantially zero internalstress and which will have a good appearance.

It is an object of the present invention to provide a method forelectroforming nickel having a substantially zero internal stress andhaving good appearance.

It is a further object of the present invention to provide a method forelectrodepositing nickel from a sulfamate bath wherein no specialorganic addition agents are employed and wherein the bath may beutilized over long periods of time.

It is another object of the present invention to provide a method forelectrodepositing nickel from a sulfamate bath wherein the bath ismaintained in a condition for producing nickel deposits having thedesired properties over long periods of time without the necessity formaking additions to the bath.

Other objects and advantages of the invention will become apparent fromthe following description.

Broadly speaking, the present invention comprises a method forelectro-depositing nickel having a controlled stress level and having agood appearance over a long period of time which comprises establishingan aqueous acid sulfamate bath containing about 55 to about 1 10 g.p.l.of nickel, up to about 90 g.p.l. sulfate ion, up to about 25 g.p.l.chloride ion, a buffering amount of boric acid, having a temperature ofabout 100 F. to about F., and passing current through said bath at acathode current density up to about 300 a.s.f., e.g., about 10 to about100 ash or' about 200 a.s.-f., while subjecting .at least a portion ofsaid bath to anodic oxidation. Advantageously, the anodic oxidationcontemplated in accordance with the invention is accomplished byproviding in the plating bath a control anode, which is insolubletherein, such as a platinum or platinized titanium or platinizedtantalum anode, and passing about 0.25% to about 4%, or, moreadvantageously, about 0.25% to about 2%, e.g., about 1% to about 2%, ofthe total plating current through said control anode. Most conveniently,a separate power supply is provided for the control anode, since controland positioning of the insoluble anode in the bath are therebyfacilitated. When the control anode has a platinum surface, it usuallyis operated at a potential of about 1.1 to about 1.6 volts as measuredby a probe and asatur-ated calomel reference electrode. As the currentpassed through the control anode is increased in proportion to the totalplating current, stress in the deposit tend to become more compressive.With regard to other practical operating factors, a reduction in bathnickel concentration, an increase in bath pH or an increase in bathchloride ion concentration within the ranges given hereinbefore willaffect the stress level in the deposit in the tensile direction. Areduction in bath temperature or an increase in cathode current densitywill also affect stress level in the deposit in the tensile directionmore strongly. For example, in the case of a bath operated at 140 F. andcontaining about 110 g.p.l. of nickel as nickel sulfamate and about 40g.p.l. boric acid at pH 4.0 with active anodes and with about 1.2% ofthe plating current supplied to a platinum control anode, asubstantially Zero stress level in the deposit was obtained at a cathodecurrent density of about 180 a.s.f. and the bath could be operatedalmost indefinitely without measurable change in bath pH. When thecathode current density was reduced to 100 a.s.f., the tress level inthe deposit was about 6,000 p.s.i. compressive and when the cathodecurrent density was increased to about 265 a.s.-f., the stress level inthe deposit was increased to about 6,000 p.s.i. tensile. A similar bathoperated at the same temperature with about 0.6% of the plating currentsupplied to a platinum control anode provided a substantially zerostress level in the deposit at a cathode current density of about .150a.s.f., whereas at a cathode current density of 100 a.s.f., the stresslevel in the deposit was about 4,000 p.s.i. compressive and at a cathodecurrent density of about 205 a.'s.f., the stress level in the depositwas about 4,000 p.s.i. tensile. Minor acid additions were neededperiodically to control bath pH during the runs made with 016% of theplating current supplied to the control anode. Another bath containingabout 80 g.p.l. of nickel as nickel sulfamate, about 40 g.p.l. of boricacid, having a pH of 4.0 and operated at 120 F. with about 1. 1% of theplating current supplied to the platinum control anode yielded asubstantially zero stress in the deposit at a cathode current density ofabout 43 a.s.f. When this bath was operated with a cathode currentdensity of about a.s.f., other conditions being the same, the internalstress level in the deposit was about 12,000 p.s.i. compressive, whereaswhen the cathode current density was increased to about 50 a.s.f., theinternal stress level in the deposit was about 3,000 p.s.i. tensile.

Advantageously, the nickel material employed at the anode in accordancewith the invention is an active nickel containing a small amount of anactivating agent such as aboutv 0.02% to about 0.04% sulfur, since suchanode materials have a limiting anode current density of about 500a.-s.f. even in chloride-free sulfamate plating bath-s such as a nickelsulfamate plating bath containing about 450 g.p.l. of nickel sulfamate,about 30 g.p.l. of boric acid and the balance essentially water. Thebath may contain chloride ion in an amount up to about g.p.l., e.g.,about Zero g.p.l. to about 5 g.p.l., as the chloride ion tends todecrease anode passivity. Chloride ion concentrations exceeding about 25g.p.l. are undesirable because internal stress in the deposit is therebyundesirably increased in the tensile direction. In producing the bathcontemplated in accordance with the invention, the content of nickelsulfamate should be at least about 300 g.p.l. (55 g.p.l. of nickel) upto about 600 g.p.l. (109 g.p.l. of nickel) to provide a bath capable ofyielding sound deposits of good appearance employing plating currents atthe cathode current densities contemplated herein. The pH of the bathmay be from about 3 to about 5 in order to avoid undesirable bathhydrolysis on the one hand and to avoid undesirable stress increases inthe deposit on the other. More advantageously, the pH is from about 4 toabout 4.5 because the pH control is then facilitated. The bath containsa buffering agent such as boric acid in amounts up to saturation, e.g.,about 25 or about g.p.l. up to about to about g.p.l. of boric acid. Thebath is operated at a temperature of at least about 100 F. up to about170 F, e.g., about 120 F. to about 140 F. This temperature rangeincludes the range of 100 F. to 120 P. which is necessary for theproduction of electrotypes and provides satisfactory plating rates andthe production of sound deposits.

The process contemplated in accordance with the invention provides aready means for controlling the stress level in the deposit. Depositsproduced at substantially zero internal stress have a bright matteappearance. Brighter deposits can be produced at more compressive stresslevels by increasing the anodic oxidation supplied to the bath. Thebrightest deposits produced without addition agents are characterized bya slight haze but are satisfactory as the basis for a chromium depositof acceptable quality. The bright matte deposits may readily be placedinto a condition for plating high quality chromium thereon by employingan intermediate conventional bright nickel layer.

It is an advantage of the invention that baths may be employed whichcontain nickel as nickel sulfate in amounts u to approximately theamount of nickel added as nickel sulfamate while still obtainingdeposits having a low stress level. Sulfate-sulfamate baths arematerially less expensive than all-sulfamate baths. Additions of sulfateion to the bath appear to move the stress level in the deposit in thetensile direction.

The invention is particularly advantageous in relation to electroformingprocesses wherein it is known that a substantially zero internal stress,i.e., an internal stress between about 1,000 p.s.i. compressive andabout 1,000 p.s.i. tensile, is advantageous. The invention accomplishesthis result and at the same time provides a nickel deposit having goodappearance. The cathode current density employed in accordance with theinvention is generally at least about 20 a.s.f. to provide an acceptableplating rate, but usually does not exceed about 200 a.s, f. because itthen becomes more difficult to produce sound stressfree deposits,particularly with the more dilute baths and the lower operatingtemperatures.

In order to give those skilled in the art a better understanding of theadvantages of the invention, the following illustrative example isgiven:

Example A bath containing about 300 g.p.l. of nickel sulfamate (about 55g.p.l. nickel ion), about 40 g.p.l. of boric acid and the balanceessentially Water was prepared. The bath had a pH of about 4 and wasoperated at a temperature of about F. An active nickel sla-b anodecontaining about 0.03% sulfur and having a surface area of about 72square inches was inserted in the bath. In addition, a platinum anodehaving an effective surface area of about 1 square inch was alsoinserted in the bath and each of the anodes was provided with a separatecurrent supply. A cathode having an effective area of about 48 squareinches Was also inserted in the bath. Current was passed through thebath at a cathode current density of about 20 a.s.f. Current was passedto the nickel anode at an impressed voltage of about 2.2 volts and theplating current was 6.6 amperes. The current supplied to the platinumanode was about 0.42% of the plating current. The bath was operated forabout 2 4 hours during which time a nickel deposit about 0.024 inchthick was produced on the cathode. The cathode was removed from the bathand the nickel deposit thereon was found to have a bright matteappearance. The deposit produced under these conditions had a zerointernal stress level by the spiral contractometer method. The bath wasthereafter operated for a total of 20,000 ampere hours and it was foundthat during the course of this time no material change in internalstress of the deposit occurred. Small periodic acid additions were madeto control bath pH during the run. When the identical bath was operatedonly with active nickel material over a period of about ampere hours,i.e., without the insoluble platinum anode, the resulting deposit wasgray and had a stress level of about 14,000 p.s.i. tensile. Again, whena similar bath was operated over a period of 20,000 ampere hours usingonly high purity (99.9%

nickel) nickel slab anode material and with a chloride ion addition tothe bath of about 5 g.p.l. to maintain anode activity, the resultingdeposit was gray and had a stress level of about 8,000 p.s.i. tensile.In each of the latter two instances, periodic acid additions were madeto control bath pH.

The theory underlying the present invention is not understood but it is,nevertheless, found that operation of the process as describedhereinbefore provides a means for electrodepositing nickel having acontrolled stress level and a good appearance over a long period oftime. The level of stress in the deposit can be controlled to besubstantially zero or to a stress level within a range from about 14,000p.s.i. compressive to about 10,000 p.s.i. tensile by appropriate controlof operating conditions. Attempts to duplicate the results obtainedthrough the controlled electrolytic oxidation of the bath as describedhereinbefore by means of chemical oxidizing agents have beenunsuccessful. Thus, it has been determined that peroxide ion, persulfateion, and dithionate ion additions to the bath have no effect. It appearslikely that the special effects yielded in accordance with the inventionare associated in some unexplained way with a special state of oxidationobtained electrolytically. Regardless of the true explanation for theunusual effects provided in accordance with the invention, the method asdescribed hereinbefore provides a means for producing nickel depositshaving controlled low stress levels and good appearance while Operatingthe bath and process over extended periods of time.

It will be appreciated that the usual impurities known to be deleteriousin nickel plating baths should be controlled to low levels in the bathoperated in accordance with the invention. These impurities includeiron, copper, zinc, lead, chromium, etc. Lead cannot be used as thematerial for the control anode contemplated in accordance with theinvention since lead sulfamate is a soluble salt. The effects ofimpurities and their removal from nickel plating baths is discussed, forexample, in the handbook Practical Nickel Plating, Second Edition, 1959,published by The International Nickel Company, Inc.

Usual oxidizable organic addition agents such as brighteners, levellers,anti-pitters, etc., become oxidized by the control anode yielding solidproducts which are removed by filtration of the bath. Such agentsaccordingly cannot be employed in the process provided in accordancewith the invention.

The control anode advantageously is platinum or a platinum-surfacedmetal such as titanium or tantalum. The control anode must act as apolarized anode in the bath and must not introduce harmful metal ionsthereinto. Carbon can be employed as the control anode, although care isneeded to remove carbon particles from the bath by filtration.Advantageously, the control anode is operated at a potential sufficientto liberate gas, e.g., oxygen. Chlorine does not form at the controlanode due to the presence of sulfamate in the bath.

It is to be appreciated that when nickel sulfamate is introduced intosolution in an amount equivalent to 55 g.p.l. nickel, about 178.5 g.p.l.of sulfamate (NH SO is introduced and that when 110 g.p.l. of nickel isintroduced as nickel sufamate, about 357 g.p.l. of sulfamate (NH SO isintroduced.

The process provided in accordance with the invention may be employed inthe electroforming upon a conductive mandrel of complex shapes such ascoffee pots and other utensils and of simpler shapes such as recordstampers and the like and can be employed in decorative nickel platingor in any other application wherein control of stress level in thedeposit is of value. The nickel deposits produced in accordance with theinvention are ductile and soft after a heating to a red heat, e.g.,about 1000 F.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

I claim:

1. The process for electrodepositing nickel having a controlled stresslevel which comprises establishing an aqueous acid sulfamate bath havinga pH of about 3 to about 5 and containing about 55 to about 110 gramsper liter of nickel, up to about grams per liter of sulfate ion, up toabout 25 grams per liter of chloride ion, a buffering amount of boricacid and a temperature of about F. to about 170 F. and passing a platingcurrent through said bath at a cathode current density up to about 300amperes per square foot from an active nickel anode to a cathodeimmersed therein While subjecting said bath to anodic oxidation bysupplying current in the amount of about 0.25 to about 4% of the platingcurrent to an insoluble anode immersed in said bath to produce a nickeldeposit having a controlled internal stress level over along period ofbath operation.

2. The process for electrodepositing nickel having a controlled stresslevel which comprises establishing an aqueous sulfamate bath having a pHof about 3 to about 5 and containing about 55 to about grams per literof nickel, up to about 90 grams per liter of sulfate ion, up to about 25grams per liter of chloride ion, about 25 to about 40 grams per liter ofboric acid, and having a temperature of about 100 F. to about F., andpassing a plating current through said bath at a cathode current densityup to about 300 amperes per square foot from an active nickel anode to acathode immersed therein while subjecting said bath to controlled anodicoxidation by supplying current in the amount of about 0.25% to about 4%of the plating current to an insoluble control anode immersed in saidbath to produce a nickel deposit having a controlled internal stresslevel.

3. The process for electrodepositing nickel having a controlled stresslevel which comprises establishing an aqueous sulfamate bath having a pHof about 4 to about 4.5 and containing about 55 to about 110 grams perliter of nickel, up to about 90 grams per liter of sulfate ion,

.up to about 25 grams per liter of chloride ion, about 25 to about 40grams per liter of boric acid and having a temperature of about 100 F.to about 170 F. and passing a plating current through said bath. at acathode current density of about 10 to about 200 amperes per square footfrom an active nickel anode to a cathode immersed therein whilesubjecting said bath to controlled anodic oxidation by supplying currentin the amount of about 0.25 to about 4% of the plating current to aninsoluble control anode immersed in said bath to produce a nickeldeposit having a controlled internal stress level.

4. The process for electrodepositing nickel having a controlled stresslevel which comprises establishing an aqueous sulfamate bath having a pHof about 4 to about 4.5 and containing about 55 to about 1.10 grams perliter of nickel, up to about 90 grams per liter of sulfate ion, up toabout 25 grams per liter of chloride ion, about 25 to about 40 grams perliter of boric acid and having a temperature of about 100 F. to about170 F. and passing a plating current through said bath at a cathodecurrent density of about 20 to about 200 amperes per square foot from anactive nickel anode to a cathode immersed therein While subjecting saidbath to controlled anodic oxidation by supplying current in the amountof about 0.25 to about 2% of the plating current to a control anodehaving a platinum surface immersed in said bath to produce a nickeldeposit having a controlled internal stress level.

5. The process for electroforming nickel having a controlled lowinternal stress level which comprises establishing an aqueous nickelsulfamate bath having a pH of about 4 to about 4.5 and containing about55 to about 110 grams per liter of nickel, up to about 25 grams perliter of chloride ion, about 25 to about 45 grams per liter of boricacid and having a temperature of about 100 F. to about 170 F. andpassing a plating current therethrough at a cathode current density ofabout 10 to about 200 amperes per square foot from an active nickelanode to a conductive electroforrning mandrel immersed therein Whilesubjecting said bath to controlled anodic oxidation by supplying currentin the amount of about 0.25% to about 2% of the plating current to acontrol anode having a platinum surface immersed in said bath to producea nickel deposit having a controlled low internal stress level.

References Cited UNITED STATES PATENTS 2,625,507 1/1953 Mayper 204-49 XR2,706,170 4/1955 Marchese 20449 XR 3,326,782 6/1967 Kendrick et a1.20449 XR 8 FOREIGN PATENTS 9/1940 Great Britain.

OTHER REFERENCES Hammond, R. A. F., Nickel Plating From SulphamateSolutions, The International Nickel C0. (Mond) Ltd., pp. 1-24, 1962.

Kendrick, R. 1., High-Speed Nickel Plating From Sulphamate Solutions,transactions of the Institute of Metal Finishing, Proceedings of the 6thInternational Conference on Electrodeposition and Metal Finishing, pp.235245, 1964.

HOWARD S. WILLIAMS, Primary Examiner.

G. KAPLAN, Assistant Examiner.

1. THE PROCESS FOR ELECTRODEPOSITING NICKEL HAVING A CONTROLLED STRESSLEVEL WHICH COMPRISES ESTABLISHING AN AQUEOUS ACID SULFAMATE BATH HAVINGA PH OF ABOUT 3 TO ABOUT 5 AND CONTAINING ABOUT 55 TO ABOUT 110 GRAMSPER LITER OF NICKEL, UP TO ABOUT 90 GRAMS PER LITER OF SULFATE ION, UPTO ABOUT 25 GRAMS PER LITER OF CHLORIDE ION, A BUFFERING AMOUNT OF BORICACID AND A TEMPERATURE OF ABOUT 100*F. TO ABOUT 170*F. AND PASSING APLATING CURRENT THROUGH SAID BATH AT A CATHODE CURRENT DENSITY UP TOABOUT 300 AMPERES PER SQUARE FOOT FROM AN ACTIVE NICKEL ANODE TO ACATHODE IMMERSED THEREIN WHILE SUBJECTING SAID BATH TO ANODIC OXIDATIONBY SUPPLYING CURRENT IN THE AMOUNT OF ABOUT 0.25% TO ABOUT 4% OF THEPLATING CURRENT TO AN INSOLUBLE ANODE IMMERSED IN SAID BATH TO PRODUCE ANICKEL DEPOSIT HAVING A CONTROLLED INTERNAL STRESS LEVEL OVER A LONGPERIOD OF BATH OPERATION.